The present applicant's international patent publications WO 95/05725 (PCT/AU94/00480) and WO 96/31098 (PCT/AU96/00178) disclose various configurations and conditions suitable for differential phase-contrast imaging using hard x-rays. Other earlier disclosures of interest are to be found in Soviet patent 1402871 and in U.S. Pat. No. 5,319,694. Differential phase-contrast imaging shows great promise for viewing the internal structure of objects for which traditional absorption-contrast radiography is of limited or no value because of very weak absorption contrast. This is the case, for example, with soft tissue within the human body.
The practical issue optimally and efficiently deriving the phase-contrast image for an object from the actual record at the detector is addressed in two related papers by Nugent et al Phys. Rev. Lett. 77, 2961-2964 (1996); J. Opt. Soc. Am. A13. 1670-82 (1996) and references therein. In these papers, it has been demonstrated that with monochromatic plane-wave x-radiation as in the configurations of WO 95/05725 and U.S. Pat. No. 5,319,694, the retrieval of phase information from measurement of the propagation of intensity can be used on treating the propagation of the modified radiation field whose characteristics reflect the phase modifying effects of the object. A two-dimensional recording of the intensity of the penetrating radiation after it has traversed the object is the result of variations in the local direction of propagation of the radiation arising from variations in local refractive index, typically an indication of a boundary or rapid variation in electron density within the object or of a thickness variation. The aforementioned articles by Nugent et al utilise a treatment of the propagation of a plane monochromatic electromagnetic wave on Maxwell's equations to derive a transport-of-intensity equation and propose solutions of this equation to derive a phase-contrast image from the intensity record. These suggested solutions to the transport-of-intensity equation involve expanding the phase in orthogonal functions. The kind of function chosen depends on the shape of the sample, and thus Zernike polynomials are adequate for a circular shape whilst a Fourier expansion is most suitable for a square-shaped sample.
The aforementioned international patent application WO 96/31098 discloses an in-line phase-contrast imaging configuration utilising a substantially point source and a two-dimensional x-ray imaging detector spaced from the object. It is demonstrated in the application that, in contrast to previous phase-contrast imaging configurations, a point source may be utilised, and moreover that the source may be broadly polychromatic provided its radiation has high lateral spatial coherence, which in practical terms indicates a maximum source diameter (s) dependent upon the source to object distance (R.sub.1). The larger the source-object distance or the smaller the source size, the greater the lateral spatial coherence (see Wilkins et al Nature384 335-8 (1996). A consequence of these disclosures in WO 96/31098 is that the approach proposed is more closely related to traditional methods used for absorption-contrast radiography and should be easier to implement than earlier proposals. This method of phase-contrast imaging is especially advantageous in the hard x-ray region where the lack of suitable lens elements make other techniques conventionally used in visible light and soft x-ray microscopy unsuitable.
It is an object of the present invention, at least in one or more embodiments, to provide a method of obtaining a phase-contrast image from a two-dimensional intensity record where the penetrating radiation substantially emanates from a point-like source. In one or more embodiments, it is a particular objective to provide a method adaptable to the extraction of phase and absorption-contrast information from radiographic images recorded with a microfocus source which need not be highly monochromatic.